Method of and apparatus for heat treating metal parts



Dec. 3, 1957 |P5EN 2,815,305

METE'IOD OF AND APPARATUS FOR HEAT TREATING METAL PARTS Filed July 20, 1955 3 Sheets-Sheet 1 AAAA 3? AMPLIFIER- Ann.

H. N. IPSEN 2,815,305

METHOD OF AND APPARATUS FOR HEAT TREATING METAL PARTS I Dec. 3, 1957 3 Sheets-Sheet 2 Filed July 20, 1955 H. N. IPSEN Dec. 3, 1957 METHOD OF AND APPARATUS FOR-HEAT TREATING METAL PARTS Filed July 20, 1955 3 Sheets-Sheet 3 Penn-2101' cmzsom AT WORK suRF-AcE. NVEQTOL mow-aid TP L w w k umn United States Patent 1 2,815,305 METHOD OF AND APPARATUS FOR HEAT TREATING METAL PARTS Harold N. lpsen, Rockford, Ill. Application July 2ft, 1955, Serial No. 523,233 8 Claims. (Cl. 148-16) This invention relates to the heat treating of parts of steel or the like in a nonoxidizing gaseous atmosphere and to control of the transfer of carbon between the atmosphere and the parts.

The primary object is to control the carbon transfer by regulating the dew point of the atmosphere in which the work is heated.

Another object is to achieve uniformity in the transfer of the carbon by maintaining substantially constant the carbon potential of the furnace atmosphere, that is, the tendency to force carbon into or withdraw the same from the work surface.

A more detailed object is to regulate the carbon potential of the heated atmosphere in response to changes in the resultant of two combined factors namely the dew point and the temperature of the atmosphere.

The invention also resides in novel structural features of the apparatus for carrying out the foregoing objects.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which:

Figure l is a schematic view and circuit diagram of an apparatus for carrying out the invention.

Fig. 2 is a fragmentary longitudinal section of the heat treating furnace.

Fig. 3 is a fragmentary perspective view of part of the control apparatus.

Fig. 4 is a fragmentary section taken along the line 4-4 of Fig. 3.

Fig. 5 is a fragmentary elevational view looking from the left of Fig. 4.

Fig. 6 is a perspective view of the heater for the electrodes of the dew point sensing mechanism.

Fig. 7 is a fragmentary elevational view of the electrodes.

Fig. 8 is a fragmentary section taken along the line 88 of Fig. 7.

The curves of Fig. 9 show the manner in which the furnace conditions change at different temperatures.

While the invention may be used to advantage in various heat treating processes including carbonitriding, neutral hardening, carbon restoration, and decarburizing, it is illustrated in the drawings and will be described herein in connection with the carburizing or case hardening of steel. It is to be understood, however, that I do not intend to limit the invention by such disclosure but aim to cover all modifications and alternative uses falling within the spirit and scope of the invention as expressed in the appended claims.

Referring now to the drawings, the workpieces 10 to be treated are mounted in the chamber 11 of a furnace heated to the proper temperature by suitable gas or electric heaters 12 disposed adjacent a so-called batlle structure 13 defining the heating chamber filled with a nonoxidizing gas of a composition suited to the treatment to be given the work. For carburizing, this gas is usually supplied from a suitable gas generator 14 and preferably delivered continuously into the furnace through a pipe 14 some of the gas being permitted to escape continuously from the chamber in which the gas is maintained in motion by a motor driven fan 15. A suitable generator for this purpose is the No. 500E endothermic generator manufactured by Ipsen Industries, Inc. of Rockford, Illinois. The workpieces are introduced into and removed from the chamber 11 through an end opening normally closed by a door 16 slidable by a power actuator 17 up and down across the chamber opening.

The carrier gas delivered by the generator 14 is the product of a catalytic reaction of air and fuel gas and includes various proportions of carbon dioxide, carbon monoxide, hydrogen, nitrogen, methane, and water vapor. In carburizing, for example, the generator is usually adjusted to produce a relatively lean mixture in respect to the reaction between the gas and the work and to add proper amounts of a hydrocarbon gas for carbon control. For various reasons, including the reactions which occur between the gas constituents and the work in the chamber 11, there is usually an increase in the dew point of the chamber atmosphere which dew point may be reduced by increasing the amount of the hydrocarbon gas in the chamber. Therefore, by regulating the admission of the hydrocarbon gas, it is possible to control the dew point within the chamber and thus produce conditions conducive to the desired treatment of the work.

The present invention aims to regulate these conditions automatically so that, in spite of inherent variations that occur in service operation of the treating furnace, a high degree of uniformity will be achieved in the carbon content of the treated work. Accordingly, the present method contemplates continuously withdrawing part of the gas from the heating chamber 11, measuring the dew point thereof, and in response to a deviation of the latter from a predetermined range or value, varying the constituency of the gas admitted to the furnace chamber in a direction to reestablish .the proper moisture content. By thus maintaining the atmosphere at a substantially constant ICQ , dew point, a uniform carbon potential for a given treating temperature will be achieved. To further improve the uniformity of carbon transfer and permit heating at different selected temperatures, the invention contemplates a novel method of correlating the dew point regulation with such temperature changes so that any desired carbon potential may be selected and maintained with a high degree of uniformity under all conditions of service operation.

To regulate the furnace temperature, the supply of energy to the heaters 12 is governed by a suitable controller 21 of well known construction adapted to turn the heaters on and off in response to closure and opening of a switch 22. Changes in the furnace temperature are sensed by a thermocouple 20 and produce a voltage signal which, in accordance with well known heat treating practice, is impressed on the input of a normally balanced network or bridge 23 including a balancing potentiometer 24 actuated-from a shaft 25 driven by a reversible electric motor 25, the selection of different temperatures to be maintained in the furnace being made by adjusting a. control potentiometer 26. The mechanism illustrated schematically is a Brown Instrument electronic recording controller known as 153010.

In operation, a change in the thermocouple voltage upsets the balance of the bridge network 23 thus producing a voltage signal which is impressed on an electronic amplifier 27 and the resulting output starts the motor 25 to move the slider 28 of the balancing potentiometer in a direction to restore the bridge balance, the motor stopping when such balance is attained. Thus the position of the slider always corresponds to the temperature prevail ing in the furnace. Whenever the slider, in moving in the temperature decreasing direction, reaches a position predetermined by the setting of the potentiometer 26 and therefore of the furnace temperature desired to be main tained, a cam 29 closes the switch 22 to turn on the furnace heaters. The latter are turned off by opening of the switch when the desired temperature has been restored and detected by the controller.

While the dew point of the furnace atmosphere may be varied in other ways as by adjusting the operation of the generator 14, it is usually preferable to increase or decrease the admission of a gaseous medium containing hydrocarbons such as propane which, at the usual heat treating temperatures, are unstable and unite with the moisture to form carbon monoxide and hydrogen thus reducing the dew point and correspondingly increasing the carbon potential of the carburizing atmosphere. The dew point regulating medium, which may be natural gas is delivered from a source 31 through a pipe 30 leading to the furnace chamber 11, the flow being increased and decreased by opening and closing of a valve 32 which may be opened by a solenoid 33 in response to closure of a switch 34. In the present instance, a very small flow of the dew point controlling gas is introduced continuously into the furnace chamber as through a normally open by-pass 32 adjusted by a valve 32 so that the flow is only a small fraction of the rate at which the primary carburizing gas from the generator 14 is supplied to the furnace. By opening the valve 32, the supply of the secondary gas is increased substantially.

The valve control switch 34 is actuated by a cam 35 on the output shaft of an electronic recording controller 36 of the same construction as the temperature responsive controller above described. As before, this controller includes a reversible motor 37 energized from an amplifier 38 and drives the cam 35 as well as the slider 39 of the balancing potentiometer 40. The control point of this network may be varied by adjusting the slider of a potentiometer 41. In such controllers, the balancing motor also moves a stylus 36 (Fig. 3) back and forth across a continuously advancing chart 36 so as to make a record 36 of changes in the controlling condition.

A voltage signal corresponding in magnitude to the prevailing dew point of the furnace atmosphere is derived from a mechanism which continuously samples the atmosphere, alternately cools and heats the sample, and measures the temperature at which condensation occurs, this being the dew point temperature. For these purposes, the gas sample is cooled to a substantially uniform temperature in being conducted outside of the furnace to a chamber 64 enclosed by a housing 65 so that the flow of the gas therein is not affected by outside air disturbances. Herein, withdrawal of the gas sample is effected .by a motor driven vacuum pump 43 communicating with the chamber 64 through a pipe 43*, the sample after being used escaping from the pump outlet. Under the vacuum created within the chamber 64 during operation of the pump, gas from the furnace chamber 11 is drawn through a pipe 44 and delivered into the chamber 64 through a nozzle 46 which is bent laterally and shaped to discharge the gas in the form of a jet or stream into an adjacent narrow gap 47 (about .002 of an inch wide) formed by the opposed edges of two electrodes 48. The latter are mounted on one side wall of the housing 65 opposite a window 65a.

In the form shown, the electrodes comprise thin semicircular plates, .001 of an inch thick, of metal such as platinum foil, cemented onto a thin plate 49 (.006 of an inch thick) of nonconducting material such as glass which in turn is fastened flat against one side of a block 56 of good heat conducing material such as copper mounted in a plate 67 forming one wall of the housing 65 and having an opening 66 through which the electrodes are exposed. Insulated conductors 50 connected to the electrodes extend outwardly through holes in the plate and the block which is about one inch in diameter and of an inch thick.

On the side opposite the electrodes, the block lies in heat conducting contact with a heater 51 which preferably is of the electric type as shown in Fig. 6 and comprises flat elements 52 of resistance material joined at opposite ends to terminals 53 and having an arcuate shape so as to follow around a stem 54 integral with and projecting from the block. The heater is clamped against the block 56 by a nut 68 seated in the housing wall and threaded onto the outwardly projecting stem 54 integral with the block. The heater is adapted to be energized from a current source 55 upon closure of a switch 55 by energization of a relay 57.

The capacity of the heater to raise the temperature of the electrodes 48 is substantially greater than the cooling capacity of a refrigerating device 58 also arranged in heat conducting relation with respect to the block 56 and capable of cooling the electrodes 48 below the lowest dew point temperature which the furnace atmosphere ever attains. This device preferably comprises a motor driven compressor unit 58 (Fig. 1) of well known construction for liquefying a refrigerant 59 (Fig. 4) which is vaporized in a casing 60. One end portion 61 of the latter encloses one end of a rod 62 of heat conducting metal, the other rod end receiving the stem 54 of the copper block 56 and thus being maintained in heat conducting relation with respect to the latter.

It will be apparent that as the electrodes 48 are cooled progressively by the refrigerating device above described, moisture will condense in the gap 47 at the dew point temperature. To detect this condition, means is provided for measuring the temperature of the block 56 and impressing a corresponding signal on the network 36. Herein, this means includes a thermocouple 71 whose junction is embedded in the block 56 closely adjacent the gap 47 and in this instance immediately behind the plate 49. The thermocouple is connected by conductors 72 to the input of the network 36 on which the voltage generated by the thermocouple is impressed continuously.

The alternate heating and cooling of the electrodes in relatively short cycles, usually about three seconds, is achieved by energizing the heater whenever the electrodes become short circuited by the condensation of moisture in the gap. For this purpose, the electrodes are included in a circuit 73 having a current source therein such as a battery 74 and the winding of a galvanometer 75 whose pointer 76 remains in the zero position shown whenever the electrodes are above the prevailing dew point temperature, there being no condensed moisture in the gap which thus offers optimum resistance. When such condensation does occur, the galvanometer is energized to swing the point away from zero and thereby move a vane 78 relative to an associated inductance coil 79. The signal is thus impressed on the input of an electronic amplifier 80 whose output energizes the relay 57 thereby closing the switch 55 to energize the heater 51.

The effective capacity of the heater is substantially greater than the cooling capacity of the refrigerating device 57 so that, within a short interval, usually about 2% seconds, the temperature of the electrodes 48 and the gap 47 will have been raised above the prevailing dew point temperature of the furnace atmosphere thereby causing the condensate within the gap to be evaporated. The resulting increase in the gap resistance moves the vane 78 reversely causing the relay 57 and the heater 51 to again be deenergized. This initiates the cooling part of another cycle and, after a short interval, usually about 2 /2 seconds, moisture will again start to condense in the gap 47 causing a sharp drop in the resistance of the latter, the heater 51 thus being reenergized.

By properly correlating the capacities of the heater and refrigerating device and by enclosing the gap and direct ing the gas sample directly onto the latter in the manner described above, it is possible to so shorten the heating and cooling cycles that the temperature of the block 56 varies only a few degrees, for example three degrees Fahrenheit. Thus, the thermocouple senses the average temperature of the block and imposes on the network 36 a temperature signal which remains fairly constant and represents an accurate measure of the prevailing dew point of the furnace atmosphere.

The sensitivity of the instrument is maintained by applying a wetting agent to the glass plate 49 in the gap 47. This agent performs two functions. First, it keeps the surface of the glass plate clean and thus prevents foreign matter from collecting in the gap and electrically connecting the electrodes 48 prematurely. Second, it causes the moisture, as it condenses on the plate, to spread into a thin film rather than forming droplets. As a result, the moisture spreads quickly from one electrode to the other so that the instrument responds almost instantaneously to the first condensation. The wetting agent preferably is a chemically inert nonionic surface active agent of either the aliphatic polyoxyethene ether type or of the alkyl aryl polyether alcohol type.

In order that the position of the driven element or balancing slider 39 of the network 36 will correspond at all times to the prevailing carbon potential of the furnace atmosphere, means are provided for impressing on the input of the network a signal which corresponds to and varies with changes in the furnace temperature. Also, this temperature may be varied under manual control by adjusting the potentiometer 26 and therefore the control point of the instrument. Since the position of the driven shaft 25 of the controller corresponds at all times to the prevailing furnace temperature, it is utilized to provide a temperature compensating signal which is added to and thus combined with the dew point signal to provide the input of the controller 36. This may be accomplished by coupling, as through suitable gearing 86, the shaft 28 to the slider 83 of a potentiometer 84 so as to provide a voltage across conductors 85 connected to the input of the network 36 including the amplifier 38. As the furnace temperature increases and decreases, the slider 83 will move to increase and decrease the compensating voltage and correspondingly atfect the position of the driven element 39 of the controller 36 for any given value of the dew point. Thus, the action of the latter, although governed primarily by the dew point signal, is modified by changes in the furnace temperature so as to produce a resultant control motion which is a true measure of the carbon potential of the furnace atmosphere. These motions are recorded at 36 on the chart 36 (Fig. 3) so that the location of the stylus 36 corresponds to the prevailing value of the carbon potential of the furnace atmosphere.

The proper relation to be maintained is shown in Fig. 9, each curve representing the dew point temperature values maintained at a particular furnace temperature in order to produce different carbon potentials which are expressed in terms of the percent of carbon absorbed at the surface of a steel workpiece under a corresponding combination of temperature and dew point. For example, if the hardened case on the work surface is to have a carbon con tent of .50 percent by heating of the work to 1700 degrees F. a dew point temperature of about 37 degrees F. should be maintained.

In another of its aspects, the invention provides novel means for disabling the automatic control when conditions precluding normal operation are encountered in service use, such for example as opening the furnace door 16 to load a new batch of workpieces. This condition is detected by closure of a normally closed switch 87 (Figs. 1 and 2) mounted on the exterior of the furnace casing and arranged to be opened by a cam 88 on the door when the latter is raised to open the furnace. In response to such closure, the automatic dew point controller is disabled for an interval which is extended beyond the reclosure of the furnace door for a period suflicient to enable normal operating conditions to be restored including the purging of the furnace of air or the burning of oil or the like off from the new workpieces. For this purpose, a timer 90 is employed to operate a switch 91 controlling the supply of electrical energy to the dew point sensing and control apparatus and to the operator of the valve 32. Preferably, the delay in restoring the automatic control is extended a fixed but selectively adjustable interval beyond the. reclosure of the furnace door, thedelay, usually sevof the dew point decreasing gas into the furnace.

' 6 eral minutes, thus being uniform for successive furnace cycles regardless of variations in the time required for reloading the furnace.

A timer suitable for this operation is manufactured by R. W. Cramer Co. of Centerbrook, Connecticut and is known as No. TH15M. This timer is adapted to measure intervals up to 15 minutes determined by the setting of a knob 92. When the switch 87 is closed by reclosure of the door, the timer is energized and operates to close the switch 91 after the lapse of the period determined by the prevailing setting of the knob 92. The automatic control is thus restored so that the dew point measuring mechanism is again operative and the supply of the dew point adjusting gas may be increased by opening the valve 32 as may be required for attaining and maintaining the selected carbon potential of the furnace atmosphere. By thus interrupting the operation of the automatic control and delaying the resumption of its operation, the deviation of the furnace atmosphere from the desired composition during unloading and reloading of the furnace is effectually minimized as is the time required for restoring the desired carbon potential.

In actual service operation of the furnace and its automatic control in casehardening, the recording controller 23 is adjusted by setting the potentiometer 26 for the temperature required for efficient carburizing of the particular work to be treated, and the controller 36 is similarly set for the carbon potential corresponding to the desired hardness of the case. Heating of the furnace is started and gas of the proper constituency from the generator 14 is admitted to the furnace at the proper rate of flow. At the same time, a small flow of the auxiliary or hydrocarbon gas is admitted through the valve 32* and combined with the main carrier gas within the furnace chamber 13.

After general equilibrium has been attained, the work to be treated is loaded into the furnace and the door thereof closed, after which the operation of the automatic controls will be delayed for an interval determined by the setting of the timer knob 92. Upon closure of the switch 91 on the power lines, the dew point mechanism will become operative, the heater 51 and the associated cooler then being effective to alternately heat and cool the electrodes 48 to first evaporate the moisture condensed in the gap 47 and then cool the electrodes below the prevailing dew point. The average temperature attained by the block during the relatively rapid cycles corresponds to the prevailing dew point of the furnace atmosphere, the thermocouple voltage thus derived being applied to the network 36. Assuming that the furnace temperature remains fixed, a change of the dew point temperature will upset the balance of the network 36 and the motor 37 will be started in a direction to correct the unbalance, the motor stopping when equilibrium is attained. Whenever, as a result of the carburizing reaction, the dew point rises above the value corresponding to the desired carbon potential as determined by the setting of the potentiometer 41, the cam 35* will be turned far enough to close the switch 34 thus opening the valve 32 to increase the flow This continues until, through the above described action of the electrodes 48, the galvanometer 75, and the thermocouple 71, restoration of the proper dew point is detected. By this time, the network 36 will have reacted and the cam 35 turned back far enough to open the switch 34 and close the valve 32.

If, as by manual adjustment of the potentiometer 26, the furnace temperature is changed, the compensating potentiometer 84 will, as a result of the ensuing rebalancing of the network 23, correspondingly upset the balance of the network 36 which, as before will be rebalanced at a different dew point temperature as required (see Fig. 9) for the maintenance of the selected carbon potential at the new furnace temperature.

This application is a continuation-in-part of my co.

7 pending application Serial No. 462,750, filed October 18, 1954.

I claim as my invention:

1. In combination with a heat treating furnace, a heater for the furnace chamber, means including a device for increasing and decreasing the flow to said chamber of a medium for decreasing the dew point within the chamber, a controller selectively adjustable for different temperatures and operable to regulate said heater and maintain the furnace chamber at the selected one of said temperatures, said controller producing an output signal corresponding in magnitude to the temperature prevailing at any time in said chamber, mechanism for measuring and sensing changes in the dew point of the atmosphere within said chamber and producing a second signal corresponding in magnitude to the prevailing value of the dew point, a normally balanced network having an input responsive to said temperature and dew point signals and having a balancing element movable in proportion to the carbon potential of said furnace atmosphere, and means controlled by the movements of said element and operable when the carbon potential falls below a predetermined value to operate said device and increase the flow of said medium to said chamber.

2. Heat treating apparatus having, in combination, a heated furnace chamber for receiving parts to be treated, means for maintaining a carburizing atmosphere therein, a pair of electrodes disposed outside of said chamber and defining a narrow gap between them, means for continuously withdrawing gas from said chamber and discharging the same in a stream into said gas, means for alternately cooling and heating said electrodes to cool the same until moisture condenses in said gap and then to heat the same until the moisture has evaporated, a source of a hydrocarbon gaseous medium capable or reducing the dew point of said atmosphere when mixed therewith, means for increasing and decreasing the admission of said gaseous medium to said chamber, a normally balanced network having a balancing element and a power actuator operable in response to unbalance of the network to move said element and rebalance the network, and mech- 'anisms separately controlling the unbalance of said network one being responsive to temperature changes of said electrodes and the other to temperature changes of said furnace chamber.

3. Heat treating apparatus having, in combination, a furnace chamber for receiving parts to be treated, a heater for said chamber, means responsive to temperature changes in said chamber to regulate the supply of energy to the heater to maintain a selected temperature therein, said last mentioned means being adjustable selectively for different temperatures, means for maintaining a carburizing atmosphere in said chamber, a pair of electrodes disposed .outside of said chamber and defining a narrow gap between them, means for continuously withdrawing gas from said chamber and discharging the same in a stream into said gap, means for alternately cooling and heating said electrodes to cool the same until moisture condenses in said gap and then to heat the same until the moisture has evaporated, a source of a hydrocarbon gaseous medium capable of reducing the dew point of said atmosphere when mixed therewith, mechanism responsive to temperature changes of said electrodes and operable when the electrode temperature rises above a predetermined range to increase the admission of said medium to said chamber and correspondingly reduce such admission when the electrode temperature decreases below such value, and mechanism responsive to temperature changes in said chamber to vary said predetermined temperature value inversely with changes in the chamber temperature.

4. The method of maintaining a substantial constant carbon potential of the atmosphere within the heated chamber of a casehardening furnace including the steps of, supplying a carburizing gas to said chamber, supplying to said chamber a gaseous medium for reducing the moisture content of said chamber atmosphere, continuously sensing the temperature in said chamber and producing a signal of corresponding magnitude, withdrawing a portion of the gas from said chamber and measuring the dew point temperature thereof and producing a signal of corresponding magnitude, and respectively increasing and' decreasing the admission of said gaseous medium to said chamber in accordance with said two signals to maintain the carbon potential of said atmosphere Within a predetermined range;

5. Heat treating apparatus having, in combination, a heated furnace chamber for receiving parts to be treated, means for maintaining a carburizing atmosphere therein, a pair of electrodes disposed outside of said chamber and defining a narrow gap between them, means for continuously withdrawing gas from said chamber and discharging the same in a stream into said gap, means for alternately cooling and heating said electrodes to cool the same until moisture condenses in said gap and then to heat the same until the moisture has evaporated, a source of a hydrocarbon gaseous medium capable of reducing the dew point of said atmosphere when mixed therewith, and mechanism responsive to temperature changes of said electrodesjand operable when the electrode temperature rises above a predetermined range to increase the admission of said medium to said chamber and correspondingly reduce such admission when the electrode temperaturedecreases below such value.

6. Apparatus for measuring the dew point of a gas having, in combination, a pair of electrodes spaced apart to define a narrow gap between them, a casing defining a chamber around said electrodes to avoid disturbances therein by air currents outside of said casing, a nozzle terminating adjacent said gap and positioned to project a stream of gas toward andinto said gap, means for flowing said gas at a uniform rate through said nozzle and out of said casing, means for alternately cooling and heating said electrodes to decrease the temperature thereof until moisture condenses in said gap and then increase the electrode temperature, and means for measuring the changing temperature of said electrodes.

7. Apparatus for measuring the dew point of a gas having, in combination, a pair of electrodes spaced apart to define a narrow gap between them, a plate of dielectric material spanning said gap, means for flowing a stream of said gas toward and into said gap, mechanism for alternately cooling and heating said plate to decrease the temperature thereof until moisture condenses on said plate in said gap and then increase the temperature until the moisture disappears, a wetting agent covering said plate across said gap to cause the moisture on the plate to spread from one electrode to the other, and means for measuring the changing temperature of said plate.

8. Apparatus for measuring the dew point of a gas having, in combination, a pair of electrodes spaced apart to define a narrow gap between them, a plate of dielectric material spanning said gap, a casing defining a chamber around said electrodes and said plate to avoid disturbances therein by air currents outside of said casing, a nozzle terminating adjacent said gap and positioned to project a stream of gas toward and into said gap, means for flowing said gas at a uniform rate through said nozzle and out of said casing, mechanism for alternately cooling and heating said plate and said electrodes to decrease the temperature thereof until moisture condenses on said plate in said gap and then increase the temperature until the moisture disappears, and means for measuring the changing temperature of said plate.

References Cited in the file of this patent UNITED STATES PATENTS 2,435,895 McIlvaine Feb. 10, 1948 2,593,313 Kamm et a1. Apr. 15, 1952 FOREIGN PATENTS 538,287 Great Britain July 28,1941 

4. THE METHOD OF MAINTAINING A SUBSTANTIAL CONSTANT CARBON POTENTIAL OF THE ATMOSPHERE WIHTIN THE HEATED CHAMBER OF A CASEHARDENING FURNACE INCLUDING THE STEPS OF, SUPPLYING A CARBURIZING GAS TO SAID CHAMBER, SUPPLYING TO SAID CHAMBER A GASEOUS MEDIUM FOR REDUCING THE MOISTURE CONTENT OF SAID CHAMBER ATMOSPHERE, CONTINUOUSLY SENSING THE TEMPERATURE IN SAID CHAMBER AND PRODUCING A SIGNAL OF CORRESPONDING MAGNITUDE, WITHDRAWING A PORTION OF THE GAS FROM SAID CHAMBER AND MEASURING THE DEW POINT TEMPERATURE THEREOF AND PRODUCING A SIGNAL OF CORRESPONDING MAGNITUDE, AND RESPECTIVELY INCREASING AND DECREASING THE ADMISSION OF SAID GASEOUS MEDIUM TO SAID CHAMBER IN ACCORDANCE WITH SAID TWO SIGNALS TO MAINTAIN THE CARBON POTENTIAL OF SAID ATMOSPHERE WITHIN A PREDETERMINED RANGE. 